Let's use algebra to solve this problem.
Let \( x \) be the number of adult tickets and \( y \) be the number of student tickets.
We have two equations based on the information given:
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The total number of tickets purchased: \[ x + y = 18 \]
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The total cost of the tickets: \[ 5x + 3y = 82 \]
Now we can solve these equations step by step.
From the first equation, we can express \( y \) in terms of \( x \): \[ y = 18 - x \]
Now we can substitute \( y \) into the second equation: \[ 5x + 3(18 - x) = 82 \]
Distribute the 3: \[ 5x + 54 - 3x = 82 \]
Combine like terms: \[ 2x + 54 = 82 \]
Subtract 54 from both sides: \[ 2x = 28 \]
Now divide by 2: \[ x = 14 \]
So, the number of adult tickets purchased is \( \boxed{14} \).
To check, we can find \( y \): \[ y = 18 - x = 18 - 14 = 4 \]
Now let's verify the total cost: \[ 5(14) + 3(4) = 70 + 12 = 82 \]
The calculations confirm that the solution is correct. Thus, they bought 14 adult tickets.